Assembling a motor so that its rotor and stator maintain a concentric relationship is essential to smooth and efficient operation of the motor. End caps of the motor casing, also referred to sometimes as end plates or bearing shields, seat the rotor assembly (rotor, drive shaft and bearings), so the mounting of the end caps is important for establishing the proper centering and alignment of the rotor within the stator.
Piloting the end cap to the stator's inner diameter is commonly used in the industry today to establish and maintain concentricity of the rotor relative to the stator. This typical motor construction is illustrated in FIGS. 1 and 2. A stator winding assembly 11 comprises conductive windings 13, which define a set of coils for some predefined number of electromagnetic poles, all wound around an annular core or frame. The stator winding assembly 11 has a stator stack length Ls and is also characterized by an inner diameter IDs of a central region into which a rotor assembly fits. An insulator 15 is used on each end of the stator core to support the windings 13 and to prevent the stator's windings from shorting to the core. A rotor assembly includes a rotor stack 17 of permanent magnets with a rotor stack length Lrp, an axial drive shaft 19, and bearings 21 and 22. The rotor assembly has an outer diameter that is slightly less than the inner diameter of the stator assembly so that the rotor fits within the stator with a small air gap 23 therebetween that allows the rotor to freely rotate on its bearings 21 about its drive axis A. Two end caps 25 and 26 are provided (end cap 25 is seen in FIGS. 2A-C) that fit over the ends of the stator winding assembly 11. There is a central opening 27 in at least one of the end caps 25 (or openings 27 and 28 in both end caps 25 and 26 as shown here) through which the drive shaft 19 projects. A space 29 is also provided in the end caps 25 and 26 to seat the rotor assembly at its bearings 21 and 22. The stator stack length Ls is slightly larger on both ends than the corresponding rotor stack length Lrp so that a surface 31 of the end caps 25 and 26 can pilot into the resulting space and fit snuggly against the inner diameter of the stator winding assembly. This guarantees that the rotor assembly will be in a concentric position relative to the stator. Once a position, the end caps 25 and 26 are often secured tightly to the stator core using screws.
However, because the stator stack must be slightly longer than the rotor to provide the stator surfaces needed to guide the end caps into position, a disadvantage is that the motor length is necessarily increased for a given torque, or seen another way, that torque is reduced for a given motor length. It would be desirable if the end caps could be positioned precisely without increasing the stator stack length.